Pulsed neutron spectral carbon/oxygen (C/O) logging is a widely used shallow measurement for time lapse reservoir saturation monitoring. Because of its shallow depth of investigation, its measurement is affected by several factors among which the near wellbore logging environment is the most important. For example, hole condition, cement quality and fluids in the wellbore have drastic effects on C/O measurement. Some of these effects, such as those due to oil holdup in the wellbore, can be corrected for in the data post-processing. Others might have a permanent imprint on the data, which cannot be removed. It is therefore extremely important to fully understand the effects of the different physical parameters within the wellbore vicinity that might be affecting the measurements. An extensive study was conducted in a field where reservoir saturation monitoring logs encountered discrepancies in water saturation between open hole and C/O logs, where the latter showed changes in saturation not supported by production history. It was found that one of the main reasons behind this discrepancy is the effect of mud filtrate invasion on the shallow C/O measurement, which varies as a function of timeand rock type characteristics. Other factors, such as wellbore fluid reinvasion effect across zones of perforation, also play a role. In this paper, it is demonstrated with field examples the effect of mud filtrate on C/O logs and define criteria used to diagnose this phenomenon. It is also highlighted other factors affecting C/O measurement such as wellbore fluid reinvasion and cement condition. Last, we present recommendations and best practices to conduct a more representative reservoir saturation monitoring.

Despite its value and importance to oilfield development and reservoir management, carbon/oxygen (CO) logs are commonly associated with significant challenges that are either related to the wellbore logging environment and/or the physics of the measurement. Shallow depth of investigation is considered the greatest challenge related to the nature of the pulsed-neutron (PN) measurement. It can imply a high degree of uncertainty on the measurement and consequently the calculated water saturation, affecting the true assessment of the reservoir fluids’ saturations, especially in challenging logging environments. In this paper we introduce and prove an innovative approach to increase the depth of investigation of the PN measurement. Currently, all PN logging tools use an electric pulsed neutron generator (PNG), or "particle accelerator" or Minitron, to probe downhole formations with 14 MeV neutrons and record the returning gamma ray signal at a shallow depth of investigation (DOI), which is generally in the range of 7 inches for C/O measurement and 12 inches for sigma measurement. In this new approach, we introduce the idea of increasing DOI of the measured gamma rays through increasing the energy level of the neutrons emitted by a PNG. To prove the concept, a computer modeling and simulation study was conducted using Monte Carlo N-Particle (MCNP) for a pulsed-neutron logging tool to determine DOI for neutron energies higher than 14 MeV. The study involved five different combinations of borehole and formation fluids. Each involved a "block" of 24 MCNP calculations. The 24 calculations inside each block represented the 24 possible combinations of 3 neutron energies (14, 20, 40 MeV), two gamma ray spectral types (inelastic, capture), and four detectors. Data simulation shows that the DOI rises substantially with energy for all tested detectors. Where the enhancement in DOI with the increase in neutron energy is more prolific in case of the inelastic measurement compared to the capture measurement. And of course the deeper the detector (further from the source) the better the DOI, although this can compromise the precision of the measurement. Yet with the recent technology advancements mainly in PNG (producing more neutron population) and GR detector technology (higher and faster count rates), this shall enhance the precision of the measurement and enable us to acquire both accurate and precise measurements at deeper detectors. This patented, innovative approach shall significantly reduce and possibly eliminate one of the main reasons behind the uncertainty of reservoir saturation monitoring using PN logs, which is shallow depth of investigation of the measurement. Having a PNG that can produce neutrons at higher energy levels compared to current industry standard shall allow a deeper, more accurate and a representative evaluation of the reservoir.

For decades, it has been affirmed that pulsed neutron (PN) spectral Carbon/Oxygen (C/O) logging is the industry's most robust salinity-independent means for reservoir saturation monitoring (RSM); yet C/O logging still comes with considerable uncertainty that has to be identified and handled with ultimate care. In this paper we investigate two main aspects of such uncertainties and showcase some recommendations to enhance the accuracy of the measurement for improved reservoir saturation monitoring. Two fundamental factors affecting C/O measurement are the type of gamma ray (GR) scintillation detector crystals used and the method for C/O spectral data processing. Currently, there are mostly six types of crystals used as GR detectors in commercial PN logging tools for routine operations. Each detector type has its advantages and limitations. With respect to data processing, the most commonly adopted method is the Windows method, due to its simplicity and statistical robustness. Whereas the Yields method is much more complicated to develop and prone to statistical variation, though it tends to provide more accurate results. Similarly, each of these two methods has its own set of advantages and disadvantages. A comprehensive study involved different logging instruments and datasets acquired under various logging environments showed that both the physical properties of the detector, as well as the characteristics of the data processing method, have to be fully considered for optimum results. The Windows method, for instance, can be adequate for detectors of statistical nature. Unlike the Yields method, which requires an optimized set of detector and tool specifications. Where for certain GR detectors, significant differences in C/O data and consequently the calculated fluid saturation were observed when processed by using the Windows and the Yields methods. C/O data processing method selection is commonly fit for purpose; yet with the continuous advancement in GR detection technology, standardization is recommended for accurate and precise log measurement. Accuracy and precision are keys to C/O logging and consequently successful reservoir surveillance and oil field management. Accordingly, a new standard RSM workflow is recommended where all available elements are properly tailored, to enhance the quality of the answer product.

Sand production is one of the major issues in unconsolidated sandstone gas and oil reservoirs. The amount of the produced sand in gas wells might vary from a few grams to a few kilograms per hour, yet it can lead to the erosion of downhole and surface equipment and to partial or complete filling of the perforations, the screens or gravel pack completions downhole, hence localizing and quantifying the amount of sand downhole is a necessity for sand production management and sand shut-off, and to update accurately the existing geomechanical models to save huge costs involved from the CT clean-out operations and the repairs caused by erosion. A newly developed downhole sand production measurement was successfully tested. The tool incorporates an innovative design that enables enhanced sensitivity to minute sand entries within the wellbore with minimal interference from the logging environment. This novel measurement was trial tested in different well completions. The carefully designed logging programs involved combining production logging tools – either conventional or array logging tools – along with the downhole sand detection tool for comprehensive log data interpretation. In this paper we illustrate with examples the results of the trial test and pinpoint the advantages as well as limitations of this new technology. Log results in the vertical gas well were generally superior to those from other wells. Where the tool illustrated high sensitivity to locating small grains of sand production across the perforated intervals, as well as providing representative qualitative assessment of downhole sand production volume. Yet, most importantly it helped obtain the optimum drawdown for the free sand rate in this well. The tool was deployed also in one deviated oil well and two horizontal gas wells. These wells are completed with smart completions, sand screens and frac ports. The results helped reveal the efficiency of the screens and the integrity issues of frac ports and determine future plan to patch the frac ports to shut-off sand production. It is generally observed that log data quality and interpretation were challenged by the downhole logging environment and the well condition during log data acquisition. The new downhole sand detection technology shows added value to allocate and qualitatively quantify sand production, check the integrity of sand screens and gravel packs, and several other applications. Candidate selection, job design and real-time log monitoring are crucial for the optimum benefit of the logging survey.

Carbon/Oxygen (C/O) log is the most commonly used measurement for reservoir saturation monitoring (RSM), especially in fresh water and mixed salinity environments. In interpreting C/O logs, oil carbon density (OCD) is a required input parameter, where a single averaging number from such as oil pressure-volume-temperature (PVT) tests is commonly used. An in-situ determined OCD, taking into account OCD variety areally as well as vertically across a reservoir, would improve the accuracy of CO RSM, the objective of this paper. In a previously published work, regions of different OCDs are identified based on available crude oil PVT data across the reservoir, and each of the regions is assigned a corresponding average OCD. Although this coarse regioning can provide improvements in determinations of oil saturation (So) from C/O logs, it can be further enhanced by taking into account variations of OCD across each region. In this paper, we discuss a new approach intended to increase the accuracy of the calculated So from C/O logging data, through the integration of a continuous oil density curve into the C/O data processing workflow. The new approach utilizes oil viscosity acquired from nuclear magnetic resonance (NMR) logs, in addition to temperature logs and PVT data, to develop a localized relationship between oil viscosity and oil density. The application of the optimum correlation shall yield an accurate oil density log, which is then used as a modular dynamic input of OCD in C/O data processing. The new workflow was applied to several wells across a heavy oil carbonate reservoir, with proven vertical change in oil properties. The comparison of the new with the original saturation profile, obtained by using the conventional C/O data interpretation workflow, showed a significant increase in accuracy. Where the new approach induced a better match to openhole – resistivity derived – water saturation log across heavy oil, with both good and moderate porosities, unperforated zones. Unlike the original data processing scheme which has usually over-estimated water saturation across the same zones, because of the lack of the required sensitivity towards the heavy hydrocarbon fraction. This new technique has been proven to closely capture the changes in reservoir oil properties, increasing the accuracy of water saturation profiling across reservoirs with varying oil properties, thus provides a means to maximize the benefit of C/O logging across reservoirs of varying hydrocarbon properties and optimize oilfield development.